Positron emission tomography and computed tomography versus positron emission tomography computed tomography: tools for imaging the lung

This article reviews the potential use of positron emission tomography (PET), alone and in combination with computed tomography, for evaluating the severity of disease in cystic fibrosis. PET scanning using injected 18F-fluorodeoxyglucose provides visual and quantitative information for the rate at which glucose is taken up by the lung, a process that should relate to the presence of inflammation and reflect the extent of the disease. The computed tomography scan gives highly accurate density and anatomic information to locate areas of inflammation seen on the PET scan, increasing the accuracy of the interpretation. Until recently, the scanners have been single systems, often located in separate hospital departments. Combined systems are now commercially available, with major advantages for patients and in the quality of analytical information obtained for interpretation by the physician. The use of 18F-fluorodeoxyglucose uptake and PET scanning has been suggested as a biomarker of progressive pulmonary inflammation in cystic fibrosis. Although promising, the data so far are limited. Further studies will be needed to validate this measurement for this purpose.     

Magnetic resonance imaging and ultrafast computed tomography in cardiac tomography

Cardiac magnetic resonance imaging and ultrafast computed tomography have been available for several years. While both modalities have become established research tools, neither has gained much acceptance in everyday clinical practice. Continued development has made both techniques easier to use and far more versatile. For patients to benefit from these improvements, it is important that clinicians become more familiar with the current state of the art.     

Ultrafast computed tomography

Ultrafast computed tomography is a developing cardiac imaging modality that can provide high temporal and spatial resolution images of the beating heart in the outpatient setting. The three-dimensional registration of these images has facilitated numerous studies showing this device to be capable of quantitating cardiac anatomy, cardiac function, and cardiac blood flow. During the past 1 to 2 years, these applications have extended into definition of right ventricular muscle mass, cardiac sources for embolization, and the possibility of diagnosing asymptomatic coronary artery disease through visualization of discrete coronary artery calcification. With respect to cardiac functional analysis, studies now suggest the application of exercise ergometry and ultrafast computed tomography imaging to define the physiologic significance of coronary stenoses. Studies involving the adaptation of indicator dilution methods to ultrafast computed tomography to quantitate regional myocardial perfusion continue to suggest this as a potential tool in the noninvasive quantitation of regional myocardial perfusion. The major limitation of this method is its availability, at present, in only a select number of major medical centers throughout the world.     

Cine computerized tomography

Ultrafast (Cine) CT scanning provides cross-sectional millisecond tomography, and therefore combines digital imaging and high resolution without the need for either cardiac catheterization or any form of ECG gating. Fifty millisecond multilevel scanning at rates of 17 scans per second allows quantitation of left ventricular (LV) function at each tomographic level during a cardiac cycle. Calculations of global and regional ejection fraction can be obtained, and in addition ultrafast CT also has the potential for providing unique data concerning regional wall thickening, mass and even regional myocardial perfusion. Interventional studies with exercise and pharmacological agents have commenced and are currently being evaluated.     

Positron emission tomography

Positron emission tomography and tracers of blood flow and of metabolism offer a most unique capability: The noninvasive study of regional myocardial metabolism and its derangements as a result of regional or global myocardial disease. Research with PET not only has confirmed the existence of metabolic fluxes and reactions as established previously through highly invasive or even destructive investigational techniques but has provided new insights into pathophysiologic processes, especially in ischemic and post-ischemic myocardium. From these investigations in both animal experiments and in humans, observations have emerged which indicate a place for PET in clinical cardiology. PET is likely to contribute to detection of disease, to characterizing its extent and severity as well as to decide upon the most appropriate therapeutic strategy and assessing its results. It is recognized that many of these observations with clinical implications await confirmation through larger clinical trials, follow-up studies as well as independent confirmation. Besides exploring ischemic heart disease, PET is equally suitable for examining substrate fluxes and interactions in other disorders as for example in intrinsic myocardial disease like primary and secondary cardiomyopathies. While derangements of metabolism in these disorders may be an expression of the consequences of the disease process or its underlying mechanisms itself, findings on PET will allow formulation of new hypotheses on disease mechanisms that conversely can then be tested. In addition to F-18 2-deoxyglucose and C-11 palmitate, the number of tracers for substrate metabolism is likely to increase. An example is C-11 acetate currently intensely investigated as a tool for measuring overall myocardial oxidative metabolism. Others as for example C-11 labeled short chain fatty acids are on the horizon. The study of cardiac receptors is similarly possible. Thus, a set of tools will soon be available for dissection of entire metabolic pathways and for determination of rate limiting steps in health and disease and to more clearly define specific defects in biochemical reaction steps that critically contribute to or even ae the specific cause of disease.     

Applied potential tomography

Applied potential tomography (APT) is a new noninvasive, nonradioactive method of measuring gastric emptying, which generates profiles of emptying of liquids that are similar to those obtained simultaneously by scintigraphy and dye dilution. This study validates the ability of APT to measure emptying of a solid beefburger test meal from the stomach by comparing the results obtained with those obtained simultaneously by scintigraphy. When acid secretion was inhibited, there was a significant correlation between the two methods for the time taken for half the meal to empty from the stomach and the amount of meal emptied at different time intervals. Furthermore, the profiles of gastric emptying obtained by APT resembled those obtained by scintigraphy in most studies. If acid secretion was not inhibited, there was no correlation between values obtained by the two methods.     

Cardiac radioisotopic tomography

In addition to positron tomography, which involves the use of radioactive elements of physiological or pathological significance, there is another technic available, that of gamma emission tomography, which makes use of conventional tracer elements, and which therefore has important clinical applications which complete plane projection imaging. This method can be used both to investigate myocardial blood supply, using thallium 201 and to evaluate cardiac kinetics--of both the right and left ventricles--after labeling of the blood flow with technetium 99m.     

Positron emission tomography

Positron emission tomography (PET) is currently the most sophisticated scintigraphic imaging technique developed for in-vivo quantification of cardiac physiology and biochemistry. The state-of-the-art PET technology allows delineation of regional tracer activity with high spatial and temporal resolution. A large number of radiopharmaceuticals have been developed to study myocardial perfusion enabling accurate diagnosis and localization of coronary artery disease (CAD) and energy metabolism. More recently, newer tracers such as radiolabeled catecholamine analogues allow the pre- and postsynaptic evaluation of cardiac autonomic innervation. Metabolic imaging with PET represents currently the gold standard for tissue viability assessment with well-validated diagnostic and prognostic information. F-18 deoxyglucose has been also used in combination with SPECT or coincidence imaging providing comparable clinical information but without need for the expensive and rarely available imaging technology of PET. The assessment of coronary flow reserve is the most sensitive scintigraphic method to i) detect vascular abnormalities before their hemodynamic significance, ii) diagnose and define the extent of CAD, and iii) to monitor the effects of (non)pharmacological intervention on regional and global cardiac flow. C-11 hydroxyephedrine (HED) allows imaging of sympathetic neuronal function. the course of cardiac reinnervation after cardiac transplantation was demonstrated with C-11 HED PET, and preliminary evidence suggests that this technique might provide prognostic information on sympathetic neuronal status in congestive heart failure, too. The functional and prognostic relevance of PET imaging together with the increased availability of lower cost instrumentation imaging will define its future role in the diagnosis, assessment of extent, prognosis and in the therapeutic decision making of cardiac disease.     

18F-NaF positron emission tomography/computed tomography in voriconazole-induced periostitis

Clinical Rheumatology -     

Airway dimensions measured from micro-computed tomography and high-resolution computed tomography.

Volume averaging results in both over- and underestimation of airway dimensions when they are measured by high-resolution computed tomography (HRCT). The current authors calibrated computerised measurements of airway dimensions from HRCT against a novel three-dimensional micro-computed tomography (CT) standard, which has a 50-fold greater resolution, as well as against traditional morphometry. Inflation-fixed porcine lung cubes were scanned by HRCT and micro-CT. A total of 59 lumen area (Ai), 30 wall area (A(aw)) and 11 lumen volume (Vi) measurements were made. Ai was measured from the cut surface of 11 airways by morphometry. Airways in scanned images were matched using branching points. After calibration, the errors of Ai, A(aw) and Vi HRCT measurements were determined. The current authors found a systematic, size-dependent underestimation of Ai and overestimation of A(aw) from HRCT measurements. This was used to calibrate an HRCT measurement algorithm. The 95% limits of agreement of subsequent measurements were +/-3.2 mm2 for Ai, +/-4.3 mm2 for A(aw), and +/-11.2 mm3 for Vi with no systematic error. Morphometric measurements agreed with micro-CT (+/-2.5 mm2) without systematic error. In conclusion, micro-computed tomography image data from inflation-fixed airways can be used as calibration standards for three-dimensional lumen volume measurements from high-resolution computed tomography, while morphometry is acceptable for two-dimensional measurements. The image dataset could be used to validate other developmental three-dimensional segmentation algorithms.     

Cranial computed tomography and magnetic resonance tomography in cerebrovascular occlusive disease

CT is the method of choice for acute diagnosis of cerebral infarction. Attention is to be paid to the alterations of the absorption values with time during the infarction course. In particular, the "fogging effect" should be considered in the second and third week when the infarct may not be demonstrable in the simple CT owing to isodensity and lack of signs of space occupation. The xenon CT enables cerebral blood flow to be measured with calculation of regional cerebral blood flow in ml/100 g brain tissue/min. The simple clinical implementation and the good anatomical correlation are advantageous. MR offers a further non-invasive possibility of diagnosing cerebrovascular diseases. MR is especially efficient in detecting small lacunar infarcts in the medullary bed and in the brain stem. The application of gadolinium-DTPA has in particular differential diagnostic significance. In MR angiography, it is to be expected that the large vessels of the neck and brain can be imaged in the future without administration of contrast media.